1. Field of the Invention
The present invention relates to a semiconductor storage unit and, more particularly, it relates to an improved semiconductor storage unit which can perform readout operation at a high speed.
2. Description of the Prior Art
FIG. 1 shows the structure of a conventional single-transistor type MOS dynamic memory of n rows and n columns. It is to be noted that circuit parts not particularly related to the present invention are omitted from FIG. 1 for convenience of illustration. In FIG. 1, memory cells 1 are arranged in the form of a matrix with n rows and n columns. The memory cells 1 in the respective rows are connected with word lines WL.sub.1 to WL.sub.n. The dynamic memory as shown in FIG. 1 is the so-called open bit type dynamic memory in which bit lines are connected to both sides of sense amplifiers 2. Therefore, the memory cells 1 of the first to (n/2)th rows are arranged in the left-hand sides of the sense amplifiers 2 while the memory cells 1 of the ((n/2)+1)th to nth rows are arranged in the right-hand sides of the sense amplifiers 2. The respective columns of memory cells 1 arranged in the left-hand sides of the sense amplifiers 2 are connected with bit lines BL.sub.1 to BL.sub.n. On the other hand, the respective columns of memory cells 1 arranged in the right-hand sides of the sense amplifiers 2 are connected with bit lines BL.sub.1 ' to BL.sub.n '. In the following description, the word lines are generally indicated by symbol WL while the bit lines in the left-hand sides of the sense amplifiers are generally indicated by symbol BL and those in the right-hand sides are generally indicated by symbol BL'.
Each of the memory cells 1 is formed by an access transistor whose gate is connected with the word line WL and a memory capacitance Cs. The word lines WL in the left-hand sides of the sense amplifiers 2 are connected to a row decoder 3 while those in the right-hand sides are connected to a row decoder 3'. These row decoders 3 and 3' are circuits which receive row addresses RA from a timing signal generation circuit 20 for selecting and driving corresponding word lines WL. The sense amplifiers 2 are interposed between the bit lines BL in the left-hand sides and the bit lines BL' in the right-hand sides. These sense amplifiers 2 are adapted to detect and amplify cell information transferred from the memory cells 1 to the bit lines BL and BL'. Column decoders 4 and 4' are adapted to receive column addresses CA from the timing signal generation circuit 20 to select the bit lines BL and BL' connected with prescribed sense amplifiers 2, and the respective outputs thereof are supplied to gates of switching transistors 9 and 9'. The switching transistors 9 and 9' are respectively interposed between the bit lines BL and BL' and I/O buses 10 and 10', thereby to connect the bit lines BL and BL' selected by selection signals from the column decoders 4 and 4' with the I/O buses 10 and 10'. The I/O buses 10 and 10' are connected to an output pre-amplifier 5, which is adapted to differentially amplify the level difference between the I/O buses 10 and 10'. The output of the output pre-amplifier 5 is supplied to an output main amplifier 6, the output of which is supplied to an output pin 11.
The I/O buses 10 and 10' are also connected with a precharging and equalizing circuit 7, which is adapted to precharge and equalize the I/O buses 10 and 10' at supply voltage V.sub.CC in advance to selection of the bit lines BL and BL' in order to facilitate high-speed readout operation. Parasitic capacitances 8 are inevitably present between the I/O buses 10 and 10' and the respective bit lines BL and BL'.
The timing signal generation circuit 20 receives a row address strobe signal RASout and a column address strobe signal CASout from the exterior and a row address signal and a column address signal inputted in a time-sharing manner from an input terminal 20a. In response to the row address strobe signal RASout and column address strobe signal CASout from the exterior, the timing signal generation circuit 20 output various timing signals or address signals to supply the same to the aforementioned respective circuit blocks.
FIG. 2 is a waveform diagram for illustrating the operation of the circuit as shown in FIG. 1. The operation of the circuit of FIG. 1 is now described with reference to FIG. 2. When the row address strobe signal RASout supplied from the exterior to the timing signal generation circuit 20 is turned to a low level, i.e., "0" in logic level, an internal inverted row address strobe signal RAS falls and a row address strobe signal RAS rises. In response to the conversion of the row address strobe signal RAS to a high level, an internal row address RA is generated and the states of the row decoders 3 and 3' are determined while one of the word lines WL.sub.1 to WL.sub.n is selected by a word line driving signal WS. Information in the memory cell 1 connected with the selected word line WL is transmitted to the bit line BL (or BL'). At this time, reference information is transmitted to the bit line BL' (or BL) on the opposite side with respect to the sense amplifier 2 from a dummy memory cell (not shown) having capacitance substantially half the memory capacitance Cs through a dummy word line (not shown). Then a sense amplifier driving signal .phi..sub.S rises whereby the sense amplifier 2 differentially amplifies a very small potential difference caused between the bit lines BL and BL'.
In the timing signal generation circuit 20, an internal inverted column address strobe signal CAS falls and a column address strobe signal CAS rises in response to the fall of the column address strobe signal CASout from the exterior. An internal column address CA is generated and the states of the column decoders 4 and 4' are determined in response to the conversion of the column address strobe signal CAS to a high level. A signal .phi..sub.Y supplied from the timing signal generation circuit 20 to the respective column decoders 4 and 4' is a column selection signal for making conductive the switching transistors 9 and 9' connected with the bit lines BL and BL' selected by the column address CA. Therefore, generation of the column selection signal .phi..sub.Y is performed by AND of the column address CA and the sense amplifier driving signal .phi..sub.S since it must be generated after the determination of the selected bit lines in the column decoders 4 and 4' and the determination of the bit line potential by the sense amplifier 2. When the column selection signal .phi..sub.Y makes prescribed bit lines BL and BL' communicate with the I/O buses 10 and 10', the sense amplifier 2 responds to the difference in potential between the I/O buses 10 and 10' precharged at the same potential level. However, such potential transition is not rapidly performed since the load capacity of the I/O buses 10 and 10' is much greater than the driving ability of the sense amplifier 2.
The precharging and equalizing operation of the I/O buses 10 and 10' is terminated when the inverted column address strobe signal CAS is turned to a low level. However, in the case where the precharging and equalizing operation of the I/O buses 10 and 10' is terminated before generation of the sense amplifier driving signal .phi..sub.S as shown in FIG. 2, the levels of the I/O buses 10 and 10' are unbalanced by capacitive coupling by the parasitic capacitance 8 following transition of most of the bit lines in one side of the sense amplifiers 2 from high levels to low levels if most of the memory cells connected to the selected word lines store the logic "0" (low level). In other words, oscillation in the output level of the sense amplifier 2 is transmitted through the parasitic capacitance 8 to the I/O bus 10 or 10', whereby the level of the I/O bus 10 or 10' is changed from the precharge potential. Therefore, the potential difference is already caused between the I/O buses 10 and 10' when the column selection signal .phi..sub.Y rises at timing t1 as shown in FIG. 2 whereby the selected bit lines are connected with the I/O buses 10 and 10'. If the potential difference is inverted in polarity from the potential difference between the bit lines BL and BL' differentially amplified by the sense amplifier 2, considerable time is required until the potential difference between the I/O buses 10 and 10' reaches a level exceeding the sensitivity of the output pre-amplifier 5 (timing t2 as shown in FIG. 2). Therefore, the timing for initiating amplification of the level difference between the I/O buses 10 and 10' in the output pre-amplifier 5, which timing is determined by rise of a driving signal .phi..sub.PA, must be held for a considerable time from the timing t1. Thus, generation of a driving signal .phi..sub.MA for the output main amplifier 6 is delayed by the said time to delay the access time.
"IEEE Journal of Solid-State Circuits" Vol. SC-15, No. 5, October 1980, pp. 846-855 is known as literature relating to coupling noise to I/O buses caused by parasitic capacitance in the operation of sense amplifiers.
In the conventional semiconductor storage unit as hereinabove described, the access time must be determined in consideration of the unbalance caused in the potential levels of the I/O buses 10 and 10' when the precharging and equalizing operation of the I/O buses 10 and 10' is terminated before driving of the sense amplifiers 2, whereby the read-out operation cannot be performed at a high speed.